Electropolishing bath



S. M. WEISBERG ETAL ELECTRO-POLISHING BATH Filed Dec. 4, 1941 BY E y ATTORNEYI tlammw UNITED STATES PATENT OFFICE ELECTROPOLISHING BATH Samuel M. Weisberg and Irvin Levin, Baltimore, Md., assignors to Sealtest, Inc., Baltimore, Md., a corporation of Maryland Application December 4, 1941, Serial No. 421,634

3 Claims.

The present invention relates to the electropolishing of metals, that is, the polishing of metals by electro-chemical processes. More particularly, the invention relates to the electropolishing of metals by immersing the metal to be polished in a suitable electro-polishing bath and using such metal as the anode in a suitable electrical circuit.

It is now common practice to polish metals electro-chemically and various methods for accomplishing this result have been devised and are -noW being used. In these existing methods, the electrolytic baths used are formulated specifically for the particular metal that is to be polished and considerable difficulty is experienced in such existing electro-chemical polishing methods, not only in effectively controlling the polishing operation but because of the danger involved in the use of the electrolytic baths.

This subject becomes of particular importance in connection with the polishing of stainless steels, where mechanical polishing is costly and difficult, and it, therefore, is highly desirable to provide an electro-chemical method of polishing such metals which is not only easily controlled but Which provides a wide latitude in the types of metals which can be polished and also enables the polishing operation to be carried on safely.

An object of theinvention, accordingly, is to provide an improved method of electro-polishing metals, as well as an improved electro-polishing apparatus and electrolyte for carrying out such a method.

A further object of the invention is to provide, in an electro-chemical process of the above character, effectively controlled steps by means of which the polishing of metals electro-chemically may be carried on under easily maintained and controlled conditions and more effectively than possible with available methods.

A further object of the invention is to provide an electro-chemical polishing bath by means of which metals such as stainless steel, mild steel, semi-steels, the noble metals and the like may be effectively polished under conditions that are easily controlled.

Yet another Object of the invention is to provide a composition of the above character which may be easily rejuvenated in order that the composition may always be available for the desired operations and in a desired condition.

Further objects of the invention will be apparent as it is described in further detail in connection with tle specific examples and illustrations hereinafter referred to, and also in connection with the accompanying drawing wherein the single figure illustrates diagrammatically one form of apparatus by means of which the invention may be practiced.

Before proceeding with a detailed description of the invention, it will be understood that, for a given electro-chemical bath, the character of the finish produced upon the metal to be polished depends upon optimum values of the bath temperature, the current densities, and the time of treatment, as well as upon the condition of the metallic surface prior to being treated. Where expedient, the surface may be subjected to a preliminary mechanical*lr polishing operation, but this, of course, may be varied in accordance with the factors controlling the individual operations, and the electro-chemical polishing may be applied to a surface regardless of the degree or extent to which it has already been polished or regardless of the condition of the surface to be polished.

We have discovered that stainless and other steels can be electro-chemically polished by means of a bath containing lactic acid, a mineral acid having a high dissociation constant, and one or more compounds which apparently form ionic complexes with the metal or act as a diluent to prevent etching of the metal by the strong acids. This polishing operation may be controlled by the current density and other factors as above enumerated.

More specifically, we have discovered that a mixture of lactic acid, sulfuric acid and phosphoric acid can be utilized to impart a glass-like nish to stainlessrsteels, mild or sc rpigsizels, and to stainless steel alloys as Well as the nob metals.

We have found that a particularly good electro-chemical bath may be formed by using lactic acid in conjunction With a mixture of phosphoric and sulfuric acids, the proportion of lactic acid being relatively low. An illustration of a particularly effective electro-polishing bath is as follows:

Example A Parts 100% total lactic acid 50 syrupy orthophosphoric acid 177 Conc. sulfuric acid 40 This formula may be varied as follows:

Per cent lactic acid (includizrjf anhydride) 15 to 40 100% sulfuric acid 5to 30 100% phosphoric acid 35to 60 Water 2 to 20 (The foregoing parts are by weight.)

tions of temperature and current density. Such baths are particularly effective in polishing mild steels at room temperatures at which the electrolyte is still liquid. This is of considerable importance because, heretofore, mixtures of citric and sulfuric acids, for example, cannot be so used.

The polishing operation on stainless steels may be effected by utilizing a current density of .5 to 2.0 amperes per square inch in anode surface being polished at a temperature of 160 to 200 F. and for a time of treatment of from two to thirty minutes. For polishing mild steels, these factors may be changed by utilizing a current density of from 0.1 to 0.7 ampere per square inch of anode surface to be polished, and optimum room temperature of from 65 to 90 F. and for a time of treatment of from one to two hours, depending upon the original condition of the metal surface to be polished.

Most of the compounds obtainable commercially for use in preparing the electrolytic baths according to the examples above stated may contain water. For example, the sulfuric acid may contain about 5% water, the phosphoric acid about 15% water, and the lactic acid may have a water content that varies from 0 to 50%. In som'e of the formulae stated in the foregoing'examples, water may be added to the electrolytic baths either to dissolve one or more of the components or to aid in supplying the proper current density. On the other hand, polishing can be obtained without the addition of any water, the formulae given in the above examples representing the optimum conditions for good polishing. It is to be understood, however, that all of these formulae may be varied within reasonable limits and the water content of the bath changed to suit the particular conditions of operation. In general, the greater the water content, the greater should be the proportion of electrolytes of low conductance such, for example, as phosphoric and lactic acids; while, Where the water content is decreased, the proportion of electrolytes of higher conductance (such as sulfuric acid) should be increased. Thus, the formulae given in the foregoing examples may be varied to include from approximately 2 to 20% of water, depending upon the conditions of the electrolyte.

An illustration of a formula giving good polishing results with a relatively large amount of water is as follows:

Example B Per cent 100% lactic acid 33 85% phosphoric acid 40 Conc. H2504 13.5 Water 13.5

' temperature of the bath may be controlled in a desired fashion over a suitable range of temperatures (for example, between 70 and 90 C.), a suitable rheostat or other device for controlling the current flowing in the electrical circuit, and a switch. An ammeter and a volt meter should be used to facilitate the proper control of the operation and, in constructing the apparatus, the electrical constants must be such as to carry the current above referred to safely. This is determined by the area of the stainless steel to be polished at one operation.

A diagrammatic illustration of suitable apparatus for carrying out the foregoing operations will be found in the accompanying drawing wherein the container or receptacle is illustrated at I, as containing the electrolyte 2. The container may be formed of wood, ceramic ware or other material that is not attacked by the acids of the electrolyte,

Within the container I, cathodes 3 and 3 are provided, the cathodes being connected by a wire 4 to a variable rheostat 5. The rheostat 5 is connected to the negative terminal of a source of electricity such as a battery 6 by means of a wire 'I and switch 8.

The positive terminal of the source of electricity 6 is connected by means of a wire 9 to an anode I0 which consists of the material to be polished. An ammeter II is connected in the circuit, for example, in the Wire 9, and a volt meter I2 is connected across the electrodes by means of a wire I3.

The temperature of the electrolyte 2 is controlled by means of a resistance I4 that is connected across the battery 6 by means of a wire I5, a variable rheostat I6 being provided to control the current iiow in the heating circuit, thus to control the temperature of the heating element I4 and the electrolyte 2. Other heating means such as steam coils and the like may be used if desired.

The cathodes 3 and 3 may be formed of inexpensive sheet metal of a thin gauge such as 24 to 30 gauge or, if desired, by a closely spaced screening, and the cathodes preferably should not be smaller than the anode. Also, it is preferable that the cathodes should follow generally the shape of the anode in order to secure optimum results. In selecting the material of the cathode, material should be chosen that is not appreciably attacked by the acid of the electrolyte when the polishing operation is not taking place. For example, copper and brass have been found to make suitable cathodes.

In polishing stainless steel, the temperature of the electrolytic bath is preferably maintained constant and in the neighborhood of C. for optimum results. A temperature variation of not more than plus or minus 10 is found not to affect appreciably the polishing effect.

As previously stated, the current density required for polishing stainless steel may range from 0.5 to 2.0 amperes per square inch of stainless steel surface being polished, 1.0 ampere per square inch usually giving optimum results. When polishing mild steels, the temperature of the electrolyte is maintained at about room temperature (20 to 30 0.), and the current density is reduced as hereinabove described. It is prefera-ble, under these conditions, to increase the electro-polishing time beyond that required for polishing stainless steels, when using, for example, the lactic-phosphoric-sulfuric electrolyte or other electrolyte described above. The time required for the electro-polishing operation depends largely upon the condition of the surface of the stainless steel anode, as well as upon the degree of polishing desired to be accomplished.

'I'his time may vary from one to twenty minutes for stainless steel. Experience has shown that a No. 4 grind nish (180 grit) requires ve minutem the polishing bath in order to secure a smooth mirror nish under the optimum conditions above mentioned. We have found that these conditions approximately prevail for all stainless steels regardless of the composition of the electrolyte.

In carrying out the-operation, it is preferable that the bath be heated to the desired temperature before the electrodes are placed in the bath. The electrodes are then inserted in the bath, the rheostat 5 adjusted to include its full resistance in the circuit and the switch 8 is then closed. The rheostat is then adjusted to supply the desired current so that the proper current density prevails in the electrolytic bath. After the desired time has elapsed, the switch 8 is opened and the stainless steel anode is removed to be rinsed thoroughly with water or immersed in an inexpensive neutralizing solution which may be any of the carbonatos or phosphates of sodium or potassium, after which the steel is rinsed with water and dried.

As previously stated, after long usage, the polishing baths above described precipitate an insoluble material which consists principally of ferrous sulphate. The precipitate collects as a grainy sludge and may be removed from time to time. 'I'he settling of this sludge does not interfere with the electrolytic polishing operation, but it is necessary to rejuvenate the bath from time to time in order that its component parts are in the correct proportion.

In order that-the bath maybe rejuvenated, concentratedY 18 ttl't'l'i in an amu'i't'equal to the equivalent amount of sulphate lost in the sludge. For example, we have found that concentrated sulfuric acid should be added in an amount substantially equal to '10% of the weight of the sludge removed. If more accurate control is desired, the electrolytic bath may be analyzed chemically for the sulphate Hamme! radical and suflcient concentrated sulfuric acid then added to restore it to its original concentration.

Another method of rejuvenating the electrolyte is to determine the current through the cell at a definite voltage across the electrodes, using standard size plates for the anodes and cathodes. When the electrolyte is i'lrst prepared, the control value for the current is determined. After long continued usage, the resistance of the electrolyte increases, causing a reduction in the current flowing therethrough. Rejuvenation is then accomplished kby adding sulfuric acid in sufficient quantities to restore the current flowing through the electrolyte to the originally determined standard.

It will be understood that proportions and concentrations of the components of the electrolytic bath may be varied considerably as indicated above and, therefore, the examples given herein should be considered as illustrative only and not as limiting the scope of the following claims.

We claim:

1. A bath for electropolishing steels consisting of about fifty parts of 100% lactic acid, about one hundred seventy-seven parts of orthophosphoric acid, and about forty parts of to 100% sulfuric acid.

2. An electro-chemical polishing bath comprising essentially between about five and thirty per cent sulfuric acid, between about fteen and forty per cent lactic acid, between about thirtyfive and sixty per cent of phosphoric acid, and between about two and twenty per cent of water, al1 proportions being by weight calculated on the basis of concentrations of said acids.

3. An electro-chemical polishing bath consisting essentially of about 33% of 100% lactic acid, about 40% of 85% phosphoric acid, about 13.5% of 95% to 100% sulfuric acid and about 13.5% of water, all proportions being by weight.

SAMUEL M. WEISBERG. IRVIN LEVIN. 

